I've recently had the annoying bug on my Plasma desktop of my mouse cursor disappearing after a couple of seconds. The only way to see the mouse was to constantly move it. Annoying. Fortunately after a day or so of Googling I found the answer. It seems to be caused by an Ubuntu app, unclutter responsible for hiding the mouse. All you need to do is uninstall it

sudo apt-get remove unclutter

Restart and you will find that your mouse remains visible. Worked for me, I hope it works for you.